We have continued our study of signals involved in T cell activation. We have shown that guinea pig (GP) peritoneal exudate lymphocytes 9 PELS) when pulsed with antigen, release a T cell growth factor, TSF (T cell stimulatory factor). TSF induces the proliferation of purified guinea pig T cells in the presence of PMA, PHA, or Con A. The factor is rapidly produced, since singificant activity is detectable after only 2 hours; activity is maximal by 18 hours and is still present at a 1/16 dilution. Since PHA and Con A can replace PMA as a comitogen for TSF, PMA is not unique. GP PELS are better producers of TSF than lymph node lymphocytes. Several lines of evidence suggest that TSF is the guinea pig equivalent of TCGF (IL-2). TSF containing supernatants have IL-2 activity when tested on the IL-2 requiring CT-6 cell line. When TSF containing supernatants were absorbed with CT-6 cells, there was a significant decrease of both TSF activity as well as IL 2 activity. Finally, purified human and mouse IL 2 have TSF activity while the macrophage product IL 1 has no TSF activity. We are in the process of biochemically characterizing and purifying guinea pig TSF to enable us to study the role of TSF in inducing the biochemical and cell surface alterations that occur in lymphocyte activation.